In modern practice of heat treatment of powdered materials extensive use is made of multiple-hearth mechanical furnaces. This type of furnace generally comprises several superimposed annular hearths encased in a metallic cylindrical shell lined on the inside with a refractory material. To replace the material being treated from one hearth onto another, there is provided a power-driven cooled shaft formed with rabbles. A modern furnace may comprise up to 16 hearths. It is usually heated by furnace gases or else by burners, which are additionally mounted on some of the hearths. Gases are discharged from the furnace through an outlet pipe provided in the upper portion thereof.
This type of furnace is used for drying and firing lime, lime sludges, magnesite, as well as for oxidizing roasting of sulphide materials. Apart from being cumbersome, the furnace is rather complicated in construction and not sufficiently leak-proof, which is largely due to the presence of mechanical rabbling mechanism mounted within the furnace. Therefore, multiple-hearth mechanical furnaces are unsuitable for heat treatment of powdered or pasty materials without access of air.
Rotary-drum furnaces are now widely employed for sintering or roasting of pasty materials. The furnace of this type generally comprises an upright metallic shell lined on the inside with a refractory material. Burner and gas outlet heads are provided to make possible the furnace heating. To replace the material within the furnace, the latter is furnished with a rabbling mechanism mounted in the furnace interior. There are also provided fuel supply means and means intended to discharge the roasted material. The furnace construction is rendered very much complicated by sealings disposed at the places of connection of the furnace rotatable and fixed members.
The furnaces of the type described above are suitable for effecting sintering of pasty boxite and nepheline materials, as well as for oxidizing roasting of sulphide materials. It is practically impossible to make rotary-drum furnaces 100% leak-proof. Because of this such furnaces are never used for heat treatment of powdered or pasty materials without air access.
Moreover, the treatment of powdered materials in rotary and multiple-hearth furnaces is accompanied by the formation of immense amount of dust, which adversely affects reliability and durability of the furnace units and members.
It is common practice to use drum-type electric resistance furnaces where heat treatment of materials is conducted in the protective atmosphere. These furnaces are intended to perform heat treatment of non-sintering and other loose materials with no tendency to adhere to the furnace walls. Such furnaces are predominantly employed for the treatment of low-dust producing materials, this being important condition for setting up the circulation of an expensive inert gas.
However, the drum-type electric resistance furnaces are unsuitable for treatment of flotation concentrates, which are finely pulverized dust-forming materials. In addition, the heat treatment of flotation concentrates results in the precipitation of moisture, as well as in the separation of decomposition products obtained in the course of pyrolysis of flotation agents, and sulphur.
If heat treatment of finely dispersed dust-forming materials is effected in the protective atmosphere, use is made of furnaces intended for fluidized-bed roasting. The furnace of this type comprises a chamber with a bottom hearth through which heated compressed air is supplied to form fluidized bed of the material being treated. The most important feature of this furnace is its hearth formed as a plate from refractory concrete with multiplicity of holes protected by mushroom-like hozzles from the penetration of material. The roasted material is discharged from the furnace under its own weight through an outlet device disposed level with the fluidized bed.
These furnaces are employed for oxidizing, reducing, sulphidizing, chloride sublimation and other types of roasting of various concentrates and ores, as well as for drying granular, pasty and liquid materials.
However, these furnaces are disadvantageous in that a considerable loss on dust, at times as high as 50 percent of the initial material, is due to take place in the course of operation. As a result, the dust-collecting system, needed for the recovery of material entrained in as streams, is rendered extremely complicated and expensive. Furthermore, the recovered material should undergo secondary roasting in a muffle furnace to be followed by the recovery of valuable components therefrom.
Dust-collecting devices make the furnace much more complicated in construction and increase its operating cost. Because of this, the powdered material is granulated prior to being subjected to heat treatment in shaft furnaces. For example, there is known a shaft furnace, described in Japan Pat. No. 2803, which is used for heat treatment of granular materials in a flow of hot gas. This furnace is made up of superimposed sections each having gas inlet pipes and outlet pipes for discharging gaseous and vaporous fumes. The furnace has a downwardly flaring shaft. The charge for the furnace should be fed in lumps only, this being necessary condition for normal operation of the shaft furnace. Thus, a heat-carrying gas agent is permitted to pass through the layer of charge continuously descending in the shaft by gravity. If the furnace is charged with powdered or pasty materials, the passage of the heat-carrying agent through the charge is rendered impossible. The charge material will unavoidably stick to the furnace with the resultant clogging of gas ducts.
The present state of the ore-mining industry is characterized by an ever increasing proportion of polymetallic ores poor in valuable components. Needless to say that these ores require preliminary treatment, such as grinding and dressing. The resultant concentrates are pasty and wet powdered materials which require heat treatment (drying, roasting) in furnaces so as to be suitable for further use.
However, known furnaces fail to meet requirements for heat treatment of dispersed materials without air access; hence is the urgent need to introduce substantial improvements into conventional furnaces or else to provide a new type of furnace for similar purposes.
Great difficulties encountered on this way originate from the necessity to provide heat treatment to an ever increasing amount of pasty and wet powdered materials, which requires additional expences for auxiliary dust-collecting equipment, material rabbling mechanisms, furnace rotating devices or means for nodulizing dispersed materials.
In is therefore an object of this invention to obviate the above disadvantages.